1. Field of the Invention
The present invention relates to a thermosensitive recording material, Particularly, to a thermosensitive recording material which shows a high optical density of colored image and excellent brightness of background area, while has an improved storage stability in both colored image and background area.
2. Description of the Related Art
In accompanied with ever-expanding requirement for information and diversified kind of information in recent yeas, a many variety of recording materials are researched, developed and used in real in the field of information recording. Among them, there are used thermal recording materials which (1) can record images thereon with a simple thermal process and (2) can save the cost in lower level and be handled with much ease as subjected to printing action with a printing machine of simple mechanism, thus being utilized in various applications including the data processing (for data outputs of a desktop calculator or computer), the medical measurement recording, the facsimile acting at low and high speeds, the automatic ticket issuing (ride tickets and admission tickets), the thermal copy machine, and the labeling in POS system.
Such thermal recording materials are essentially desired to develop high optical density of color image with speedy processing and to prolong both life of image and life of background area.
Among those for satisfying that purposes, in particular, for developing high optical density of color image even if in quick development (=to show high sensitivity), additives for the thermosensitive material have been studied which may compose an eutectic mixture in jointing with leuco dye as colorant or electron-acceptor compound as color developer used in thermosensitive material to lower the melting point of the mixture. It can say that the lowering of melting point of the colorant and/or the color developer are basically efficient for increase sensitivity, however causes inevitable drop of coloring temperature, therefore, such kind of improvement in sensitivity shall concur a tinting(fogging) of background area at low temperature. Furthermore, from the point of view of improving the resistibility for affection of chemicals, there have been recently proposed color developers which are increased in the molecular weight (WO 99/51444, WO 00/14058 and Japanese Laid-open Patent Publication of Tokkai Hei 8-333329). Those color developers having high molecular weight are however still insufficient in the sensitivity and the optical density of colored image, thus higher sensitizing having been desired.
On the other hand, as other means for increasing the sensitivity of thermosensitive recording materials, various attemps have been made for giving smaller particles of leuco dye to achieve higher sensitivity with the recording materials. For example, the use of a leuco dye having average volumetric particles diameter not higher than 2 μm (Japanese Laid-open Patent Publication of Tokkai Shou 57-47693), the use of mixture consisting of two or more different type of leuco dyes which being pulverized jointly and have average particle diameters not higher than 2 μm (Japanese Laid-open Patent Publication of Tokkai Hei 7-223375), and a thermosensitive recording member using a leuco dye having an average particle diameter of 0.1 to 0.8 μm, a developer and a colloidal silica (Japanese Laid-open Patent Publication of Tokkai Hei 7-186527), and the like are instanced.
Those pulverization(in other words milling) processes to make a desired size of the leuco dye particles may be conducted using a cellulosic polymer material soluble in water such as polyvinyl alcohol, and the dispersing action of a dispersing machine, such as a ball mill, a sand mill, a high-speed jet mill, or an attrititon apparatus. However the pulverization processes have problems that more labor, time and machine are required for decreasing the size of leuco dye particles hence resulting cost elevation, and secondary aggregation is likely to occur in liquid thus declining stability of the liquid, particularly in case of being pulverized to the size less than or equal to 0.3 using water soluble polymer material, because a great many of activated surfaces are presented at dye particles due to cleavage and abrasion thereof, and such activated surfaces of dye particles and prolonged period of pulverizing process may generate a tinting or fogging of dye-dispersion liquid hence lowering the brightness of background area of thermosensitive member prepared using the dye-dispersion liquid.
To evade above problems pertinent to mechanical dispersion process, there are provided methods for making smaller particles of leuco dye using emulsification process.
For examples, disclosed in Japanese Laid-open Patent Publication of Tokkai Shou 61-2187283 concerns to a method of emulsifying liquid of oil in water type in which a solution of an organic solvent containing a leuco dye dissolved therein is emulsified. The method however requires a step of removing the organic solvent after the finishing emulsification and the remained the organic solvent may develop an affect of background density(fogging or tinting). Disclosed in Japanese Laid-open Patent Publication of Tokkai Shou 56-164890 discloses another method of emulsifying a leuco dye and a thermally fusible material jointly, however it lowers the melting point of the thermosensitive material thus causing the degradation of thermal stability during storage period. Disclosed in Japanese Laid-open Patent Publication of Tokkai Hei 7-186531 is a thermosensitive recording material which contains a leuco dye having an average volumetric particle diameter not higher than 0.3 μm and an ultraviolet ray absorbant, in which the preparing of small particles is carried out by heat-fusing the leuco dye then emulsifying the resultant by a high-pressure homogenizer with use of a silicon emulsifier.
Disclosed in Japanese Laid-open Patent Publication of Tokkai Hei 7-223379 concerns a leuco dye having an average particle size not higher than 1.0 μm which is being emulsified and dispersed, in which the leuco dye is obtained by heat-fusing a leuco dye, then emulsifying and dispersing the resultant. In general, however the melting point of leuco dye is higher than 150° C., therefore as shown in above disclosed in Japanese Laid-open Patent Publications of Tokkai Hei 7-186531 and Tokkai Hei 7-223379, the heat-fusion and emulsification thereof in water can be conducted by use of a particular system such as a high-pressure container and the like, thus a mass-production thereby will be implemented with much difficulty.
With regard to the supporting substrate for thermosensitive material, besides the use of general wood free paper, there are known the uses of synthetic fiber paper, a plastic film and the like, it has been however required in recent years that the used papers are recovered and re-used from the point of view of resources saving. Disclosed in Japanese Laid-open Patent Publications of Tokkai Shou 58-25986 and Tokkai 2000-272248 are a thermosensitive recording paper which employ a paper board substrate containing a pulp recovered from used paper and thereon is applied a thermosensitive layer. However, the use of supporting substrate containing a pulp recovered from used paper causes a problem that stability during storage is declined.